bigerror-derive 0.1.0

use proc_macro2::TokenStream;
use quote::quote;
use syn::{Data, DeriveInput, Error, Fields};

use crate::attributes::Attributes;

pub fn thin(input: &DeriveInput) -> Result<TokenStream, Error> {
    let ctx = &input.ident;
    let attributes = Attributes::parse(input)?;
    match &input.data {
        Data::Struct(data_struct) => {
            if !matches!(data_struct.fields, Fields::Unit) {
                return Err(Error::new_spanned(
                    &input.ident,
                    "struct must be a zero-sized-type to implement `ThinContext`",
                ));
            }
        }
        Data::Enum(_) => {
            return Err(Error::new_spanned(
                &input.ident,
                "enum `ThinContext` derives are currently unsupported",
            ));
        }
        Data::Union(_) => {
            return Err(Error::new_spanned(
                &input.ident,
                "union `ThinContext` derives are currently unsupported",
            ));
        }
    }

    let bigerror = attributes.crate_path;
    let impl_display = (!attributes.has_display).then(|| {
        quote! {
            impl ::core::fmt::Display for #ctx {
                fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
                    f.write_str(stringify!(#ctx))
                }
            }
        }
    });
    Ok(quote! {

        impl ::core::error::Error for #ctx {}
        impl ::core::fmt::Debug for #ctx {
            fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
                f.write_str(stringify!(#ctx))
            }
        }

        #impl_display
        impl #bigerror::ThinContext for #ctx {
            const VALUE: Self = #ctx;
        }
    })
}